Cleanup: Pass std::string as const reference from content/

Cleanup: Pass std::string as const reference from content/

Passing std::string by reference can prevent extra copying of object.

BUG=367418
TEST=
R=jam@chromium.org,jochen@chromium.org,kinuko@chromium.org

Committed: https://crrev.com/7cf10ca7017bed4b6fac3596b9b9080bf208c7d4
Cr-Commit-Position: refs/heads/master@{#351007}

[jochen (gone - plz use gerrit), 2015-09-21 15:03:26]

tryrun please

[ki.stfu, 2015-09-21 19:04:18]

The CQ bit was checked by ki.stfu@gmail.com to run a CQ dry run

[commit-bot: I haz the power, 2015-09-21 19:04:51]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1354363002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1354363002/1

[commit-bot: I haz the power, 2015-09-21 19:04:52]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-21 19:04:53]

Dry run: No L-G-T-M from a valid reviewer yet. Only full committers are
accepted.
Even if an L-G-T-M may have been provided, it was from a non-committer,
_not_ a full super star committer.
See http://www.chromium.org/getting-involved/become-a-committer
Note that this has nothing to do with OWNERS files.

[ki.stfu, 2015-09-21 19:18:13]

On 2015/09/21 15:03:26, jochen wrote:
> tryrun please

Sorry, I'm a newbie here. How I can do it?

[Adam Rice, 2015-09-22 03:22:39]

The CQ bit was checked by ricea@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-22 03:22:53]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1354363002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1354363002/1

[commit-bot: I haz the power, 2015-09-22 04:26:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-22 04:26:35]

Dry run: This issue passed the CQ dry run.

[ki.stfu, 2015-09-24 18:06:43]

On 2015/09/22 04:26:35, commit-bot: I haz the power wrote:
> Dry run: This issue passed the CQ dry run.

For the last time I saw many false negative tryjobs, especially on
win8_chromium_ng build bot. I suppose here is another one. Can somebody re-run
it please? And how does this CL look?

[no sievers, 2015-09-24 18:07:33]

The CQ bit was checked by sievers@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2015-09-24 18:08:37]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1354363002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1354363002/1

[commit-bot: I haz the power, 2015-09-24 19:44:23]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-09-24 19:44:24]

Dry run: This issue passed the CQ dry run.

[Adam Rice, 2015-09-25 05:53:34]

lgtm (but you still need lgtm from an OWNER).

In C++ there exist edge-cases where this transformation is unsafe, but all the
changes in this CL  look safe to me.

[jochen (gone - plz use gerrit), 2015-09-25 11:17:38]

lgtm

[kinuko, 2015-09-26 15:24:38]

lgtm

[ki.stfu, 2015-09-27 07:12:33]

The CQ bit was checked by ki.stfu@gmail.com

[commit-bot: I haz the power, 2015-09-27 07:12:46]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1354363002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1354363002/1

[commit-bot: I haz the power, 2015-09-27 08:37:14]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-27 08:38:05]

Patchset 1 (id:??) landed as
https://crrev.com/7cf10ca7017bed4b6fac3596b9b9080bf208c7d4
Cr-Commit-Position: refs/heads/master@{#351007}

[ki.stfu, 2015-09-27 08:51:25]

On 2015/09/27 08:38:05, commit-bot: I haz the power wrote:
> Patchset 1 (id:??) landed as
> https://crrev.com/7cf10ca7017bed4b6fac3596b9b9080bf208c7d4
> Cr-Commit-Position: refs/heads/master@{#351007}

CQ shows 2 failures, but I can't reproduce it on my side:
```
$ ./browser_tests --gtest_filter=FirstRunMasterPrefsImportDefault.ImportDefault
IMPORTANT DEBUGGING NOTE: each test is run inside its own process.
For debugging a test inside a debugger, use the
--gtest_filter=<your_test_name> flag along with either
--single_process (to run the test in one launcher/browser process) or
--single-process (to do the above, and also run Chrome in single-process mode).
Using sharding settings from environment. This is shard 0/1
Using 1 parallel jobs.
Note: Google Test filter = FirstRunMasterPrefsImportDefault.ImportDefault
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from FirstRunMasterPrefsImportDefault, where TypeParam = 
[ RUN      ] FirstRunMasterPrefsImportDefault.ImportDefault
[5496:5496:0927/101053:WARNING:password_store_factory.cc(334)] Using basic
(unencrypted) store for password storage. See
http://code.google.com/p/chromium/wiki/LinuxPasswordStorage for more information
about password storage options.
[       OK ] FirstRunMasterPrefsImportDefault.ImportDefault (1995 ms)
[----------] 1 test from FirstRunMasterPrefsImportDefault (1995 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (1995 ms total)
[  PASSED  ] 1 test.
[1/1] FirstRunMasterPrefsImportDefault.ImportDefault (2356 ms)
SUCCESS: all tests passed.
ilia@HHU:~/p/chromium/src/out/Release$ ./browser_tests
--gtest_filter=FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0
IMPORTANT DEBUGGING NOTE: each test is run inside its own process.
For debugging a test inside a debugger, use the
--gtest_filter=<your_test_name> flag along with either
--single_process (to run the test in one launcher/browser process) or
--single-process (to do the above, and also run Chrome in single-process mode).
Using sharding settings from environment. This is shard 0/1
Using 1 parallel jobs.
Note: Google Test filter =
FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0
[==========] Running 1 test from 1 test case.
[----------] Global test environment set-up.
[----------] 1 test from
FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences
[ RUN      ]
FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0
[5609:5609:0927/101102:WARNING:password_store_factory.cc(334)] Using basic
(unencrypted) store for password storage. See
http://code.google.com/p/chromium/wiki/LinuxPasswordStorage for more information
about password storage options.
[       OK ]
FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0
(1478 ms)
[----------] 1 test from
FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences
(1478 ms total)

[----------] Global test environment tear-down
[==========] 1 test from 1 test case ran. (1478 ms total)
[  PASSED  ] 1 test.
[1/1]
FirstRunMasterPrefsWithTrackedPreferencesInstance/FirstRunMasterPrefsWithTrackedPreferences.TrackedPreferencesSurviveFirstRun/0
(1834 ms)
SUCCESS: all tests passed.
```

Why it was committed if there are a few failures?

[Adam Rice, 2015-09-28 05:21:34]

On 2015/09/27 at 08:51:25, ki.stfu wrote:
> Why it was committed if there are a few failures?

It looks to me like the tests passed the final time they were run.
It's not uncommon to see a few failures, because either something
is broken for unrelated reasons at HEAD, or because the tests
themselves are flaky.